<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        div{
            width:100px;
            height:100px;
            /* ctrl+/ */
            /* border-width:1px;
            border-style: solid;
            border-color: #333;  */
            border:1px solid #333;
            /* 让一个元素实现水平居中 */
            margin-left: auto;
            margin-right: auto;
        }
    </style>
</head>
<body>
    <!-- ctrl+/ 注释 -->
    <!-- 标签名+tab -->
    <div></div>
</body>
</html>